1. Field of the Invention
The present invention is broadly directed towards new methods of forming microelectronic structures where extremely thin photoresists layers can be utilized.
2. Description of the Prior Art
When a photoresist is used to generate patterns by light exposure, Rayleigh's laws can be used to define the pattern resolution and depth of focus (DOF):Resolution=k1λ/NA; andDOF=k2λ/NA2,where λ is the irradiation wavelength, NA is the numerical aperture of the exposure tool, and k1 and k2 are constants for a given process. Rayleigh's theory indicates that an exposure tool with short wavelength and large numerical aperture will produce better pattern resolution. This principle is why the microelectronics industry has been progressively moving toward short exposure wavelengths. However, Rayleigh's law also shows that enhancing resolution causes the DOF to decrease.
The use of a thin photoresist decreases the value of k1 and increases the value of k2, which results in better resolution and a large DOF. As a result, extensive research has been conducted in an attempt to reduce photoresist thicknesses in photolithographic processes. Due to the different etch selectivities (with the different etch chemistries) of the hard mask over the photoresist, and of the hard mask over the substrate, the most effective way to reduce photoresist thickness is by introducing a hard mask into the etch stack.
Silicon nitride, silicon oxynitride, silicon nitride/oxide, silicon oxide/nitride, transition metals, amorphous silicon, and metal/oxide hard masks have been introduced into the etch stack to attempt to decrease the necessary photoresist thickness in photolithographic processes. The hard mask-to-photoresist etch selectivity for a given thickness of the hard mask determines how thin the photoresist can be. Unfortunately, organic photoresists etch relatively fast under common hard mask plasma etch chemistries and conditions. Thus, the photoresist thickness is still substantial even with multiple hard masks.
A multilayer or composite photoresist process has also been attempted. In this process, a somewhat thinner photoresist layer is formed on a substrate and is subsequently exposed and developed to pattern the photoresist. The process is repeated until the desired number of photoresist layers are accumulated. The thinner each layer is, the more layers that must be processed for a specific final thickness. The drawbacks of this process are as clear as the advantages. In particular, the procedure is cumbersome, and the patterns for each photoresist layer must be perfectly aligned.
There is a need for a process that minimizes or even eliminates these problems.